Rolling element bearings are precision made components the stiffness of which is predictable in more than one direction such that the movement of the inner ring of the bearing with respect to the outer ring is predictable as a function of load, temperature and thermal parameters.
It is known to measure loads acting on bearings using strain gauges applied to bearing. The attachment of the strain gauges entails difficulties in the manufacturing. Load measurements on bearings require gluing of strain gauges to the bearing surfaces. This is time-consuming, costly and difficult to automate. Further, it is known to measure loads using Eddy Current Sensors. This solution is fairly expensive.
Wheel bearings specifically have both rows very close to each other, so the contact lines overlap each other inside the steel structure. At the measurement position, the deformations from both rows overlap. Therefore, one cannot tell which row of the bearing is loaded. Hence, it is not possible to tell if the vehicle is cornering left or right.
For rotating outer ring bearings, the strain signal would have to be transferred wirelessly.
Methods employing non-contact sensors such as magnetic sensors are known in the prior art, e.g. disclosed in the document JP 2008-215977 A.
In this technology, a sensor system optimized for determining a wheel speed for ABS (Anti-lock Braking System) or slip control employing the frequency of the sensor signal is used to additionally determine the load acting on the bearing. For this purpose, the amplitude of the signal obtained by the magnetic sensor is read out. The amplitude of the magnetic field acting between a magnetic sensor and an angle target ring depends on the axial distance between these elements and is used to determine the relative axial position of the rings.
Commercially available wheel hub units with sensors are optimized for wheel speed detection and the period of the magnetically active pattern on or in the target ring is about 7 mm or more. This period will also be referred to as the “wavelength” of the target ring in the following. For typical target ring diameters of 30 mm or more, this pitch leads to a sufficient angular resolution and it is ensuring a sufficiently high signal-to-noise ratio and neatly detectable pulses. The relatively large wavelength ensures a good signal to noise ratio within the elastic movement of the bearing under any load and within the practical distance variation due to placing tolerances of the sensor, but does not allow a very accurate sensing of the load as it varies relatively little with the displacement between the magnetic ring and the magnetic sensor. A shorter wavelength will allow an improved change with distance.
When using standard target wheels for load detection as disclosed e.g. in JP 2008-215977 A, the characteristic function describing the dependency of the signal amplitude on the distance has a fairly shallow slope such that the resolution in the distance detection is low. The reason for this is obvious: since standard sensor wheels are optimized for wheel speed detection, the signal should be unaffected by variations in the distance as far as possible.
On the other hand, the application PCT/EP2010/00345 discloses a method employing strain gauges attached to the inner ring or to the outer ring of the bearing in order to measure elastic deformations of the bearing. The average local deformation is a measure of the load variation with respect to a baseline. However, the baseline depends on temperature and thermal gradients induced by friction heat such that it is normally impossible to derive absolute loads from absolute strains.
As a consequence, both methods are unsatisfactory. While the displacement sensor method is unable to account of effects of local strains or thermal expansions, the conventional method using strain gages has problems in determining absolute values for the load.
The document EP 1 672 372 A1 teaches to detect the load of a rolling bearing using signals of sensors interacting with encoders. The document EP 1 130 362 A2 teaches a method of manufacturing a magnetic encoder with a pitch of 1.5 mm. The document US 2008/243427 A1 discloses an encoder ring having a magnetic pattern which changes over a width of the encoder ring such that a displacement of the encoder ring parallel to its width direction results in a change of the offset of a signal detected by a sensor. The document FR 2 794 504 A1 discloses a further example of a sensorized rolling element bearing.